Fotoninių kristalų šviesolaidžių dispersijos charakterizavimas ir superkontinuumo generacijos tyrimas

Photonic crystal fibers (PCFs) are unique media for supercontinuum generation: the ability to engineer PCF dispersion, nonlinearity, birefringence, etc. by changing structural parameters of the microstructure region has given a significant impulse to supercontinuum generation research. This doctoral dissertation aims at developing novel photonic crystal fiber dispersion characterization methods and investigation of supercontinuum generation in highly nonlinear polarization-maintaining PCF with two zero dispersion wavelengths using chirped femtosecond or subnanosecond pulses. The first two chapters of this doctoral dissertation address the important issue of PCF dispersion characterization. Novel experimental method for PCF group velocity dispersion (GVD) characterization is demonstrated. Moreover, to the best of our knowledge, the first experimental method for PCF fundamental mode phase refractive index measurement is demonstrated. The third chapter of this doctoral dissertation presents a comparative study of supercontinuum generated in the aforementioned PCF using chirped pump pulses and bandwidth-limited pump pulses at the same peak power. In the final chapter, supercontinuum extension in the whole visible range and beyond using subnanosecond pump pulses is demonstrated in the same PCF without any sophisticated technological PCF modifications or complicated experimental setups. Moreover, using streak camera we could observe how a portion of supercontinuum radiation propagates not in the fundamental PCF spatial mode

Characterization of photonic crystal fiber dispersion and investigation of supercontinuum generation

Photonic crystal fibers (PCFs) are unique media for supercontinuum generation: the ability to engineer PCF dispersion, nonlinearity, birefringence, etc. by changing structural parameters of the microstructure region has given a significant impulse to supercontinuum generation research. This doctoral dissertation aims at developing novel photonic crystal fiber dispersion characterization methods and investigation of supercontinuum generation in highly nonlinear polarization-maintaining PCF with two zero dispersion wavelengths using chirped femtosecond or subnanosecond pulses. The first two chapters of this doctoral dissertation address the important issue of PCF dispersion characterization. Novel experimental method for PCF group velocity dispersion (GVD) characterization is demonstrated. Moreover, to the best of our knowledge, the first experimental method for PCF fundamental mode phase refractive index measurement is demonstrated. The third chapter of this doctoral dissertation presents a comparative study of supercontinuum generated in the aforementioned PCF using chirped pump pulses and bandwidth-limited pump pulses at the same peak power. In the final chapter, supercontinuum extension in the whole visible range and beyond using subnanosecond pump pulses is demonstrated in the same PCF without any sophisticated technological PCF modifications or complicated experimental setups. Moreover, using streak camera we could observe how a portion of supercontinuum radiation propagates not in the fundamental PCF spatial mode

Partially coherent UV–VIS light generation in photonic crystal fiber using femtosecond pulses

Light generated in optical fibers due to various nonlinear processes often has a broad spectrum and is commonly considered as a potentially convenient source of seed for tunable wavelength laser systems and many other applications. The nonlinear processes usually considered when discussing this spectrum broadening are coherent. In this paper we present experimental and theoretical investigation of a partially coherent nonlinear phenomenon occurring at the same time — ultraviolet–visible light (375 nm–500 nm) generation in a short photonic crystal fiber pumped by ∼ 110 fs duration and 1028 nm wavelength pulses

All-solid-state post-compression of low-energy pulses at high repetition rate /

We demonstrate a proof of principle of a simple all-solid-state post-compression setup for low-energy, high-repetition-rate laser pulses, where spectral broadening was performed using a combination of highly nonlinear bulk materials in a simple single-pass geometry. The 75 fs, 210 nJ pulses from an amplified 76 MHz, 15.7 W Yb:KGW oscillator after sequential spectral broadening in ZnS and YAG samples of 2 mm and 15 mm thickness, respectively, were compressed to 37 fs by means of Gires–Tournois interferometric mirrors. The post-compressed pulses with an average power of 11.47 W demonstrated reasonable spatial-spectral homogeneity of the beam with the spectral overlap parameter V > 83% and good beam quality with M2 x = 1.28 and M2 y = 1.14

Investigation of laser-induced damage and related multiphoton absorption changes in lithium niobate crystals at high repetition rate femtosecond pump

An R-on-1 laser-induced damage-threshold (LIDT) testing method was applied to test coated and uncoated magnesium oxide-doped periodically poled lithium niobate samples pumped by femtosecond Yb:KGW laser pulses at kilohertz and megahertz pulse repetition rates. LIDT values decreased by ∼1.5 and ∼38 times when increasing repetition rate from 100 to 571 kHz and to 76 MHz, respectively. We also investigated nonlinear absorption changes in lithium niobate crystals at the pulse repetition range from 60 to 600 kHz with trains consisting of 100 identical femtosecond pulses. Laser beam transmission in the crystal experienced a drop of ∼18% from initial pulses of train to the next 40 pulses at intensities 40% to 15% lower than LIDT due to nonlinear absorption of 220 fs duration pulses at 1.03 μ

Performance investigation of high-efficiency widely tunable subnanosecond optical parametric generator and amplifier based on MgO:PPLN

We report on experimental and theoretical studies of widely tunable high-efficiency subnanosecond optical parametric generator (OPG) and amplifier (OPA) based on a 2 cm long multigrating MgO-doped periodically-poled lithium niobate (MgO:PPLN) crystal pumped by a passively Q-switched Nd:YAG micro-laser. Our OPG can be continuously tuned from 1442 nm to 4040 nm with signal wave energies ranging from 33 J to 265 J and total OPG conversion efficiency up to 46 % that depended on the pump focusing conditions. Characterization of spatial properties of the OPG determine Lorentzian spatial profile of the signal beam with 2≈2 that was also dependent on the pump focusing conditions. High OPG gain and subsequent pump depletion led to the adjustment of the output signal pulse duration in the range of 242 – 405 ps by varying the incident pump power. By using a distributed feedback (DFB) continuous-wave (CW) 1550 nm wavelength seed laser for the OPA operation we reduced the generation threshold up to 1.6 times, increased maximum conversion efficiency by 4 – 20%, and achieved nearly transform-limited output signal pulses. Experimentally measured characteristics were supplemented by numerical simulations based on the quantum mechanical model for the OPG, and classical three-wave interaction model for the OPA operation